1. Field of the Invention
The present invention relates to a toner for two-component developer that is used for developing an image in a printer, an electro-photographic apparatus and the like, and a color image forming apparatus for obtaining color images using the toner for two-component developer.
2. Description of the Related Art
In a quadruple tandem type color image forming apparatus for obtaining full color images on sheets of paper, OHP films and the like that are image transferred media, by superposing and fixing toner images in plural colors, it is demanded to get good color reproducibility and also stabilized developer images even in the repetitive image forming.
In a developing device of this sort of full color image forming apparatus, if its developing system is of a one-component magnetic type, a toner containing dark black colored magnetic powder must be used and therefore, such a problem was caused that color reproducibility is worse. Further, if a single-component non-magnetic type developing system is used, although magnetic powder is not contained in a toner core, the toner is electrified between a developing sleeve and a blade through the sliding contact. Therefore, such a problem was caused that the developing sleeve and the blade were stuck by a resin if a soft and quick melting resin having the characteristic of excellent color reproducibility was used. Because of this, a two-component developing system to develop images using a two-component developer comprising toner and carrier by a magnetic brush developing device was so far generally adopted.
For raw material of toner using such a two-component developing system, a quick melting type resin is demanded from the viewpoint of color reproducibility or transparency when images are formed on OHP sheets. So, linear polyester that is a representative quick melting resin has been so far used as a principal component of toner mother particles, and coloring agents and a charge control agent are contained in this linear polyester.
The above-mentioned toner mother particles using the linear polyester as resin have a quick melting characteristic and are excellent in color reproducibility and transparency; however, as being soft, they are inferior in fluidity or caking resistance. Accordingly, in the repetitive image formation, the quality of image is deteriorated as the developing density drops or the fog is generated. On the other hand, fixing and cleaning properties are impaired and the offset to the heat roller tends to occur at the time of fixing. By oil applied to prevent the offset, the quality of image was impaired or the photosensitive drums became dirty due to the insufficient cleaning. Furthermore, as the acid value of the linear polyester is low, the charging function drops due to the repetitive image forming and the electrification became insufficient when repeating the image formation, the fog was generated or the circumference was contaminated by scattering toner.